1. Field of the Invention
The present invention relates to a nonaqueous electrolyte secondary battery wherein the nonaqueous electrolyte contains a sultone compound having unsaturated bonds.
2. Description of the Prior Art
In these years, owing to the advance in electronic technology, there have been promoted the performance enhancement and miniaturization of electric appliances such as cellular telephones, notebook-type personal computers, video cameras, etc., and accordingly, there is a very strong demand for batteries of high energy density that can be used in these electric appliances. A representative battery that can meet such a demand is a lithium secondary battery in which lithium is used as a negative active material.
A lithium secondary battery comprises, for example, a negative plate comprising a current collector supporting a carbon material which absorbs and releases lithium ions, a positive plate comprising a current collector supporting a composite lithium oxide such as a lithium-cobalt composite oxide which absorbs and releases lithium ions, and a separator holding an electrolyte solution dissolving such lithium salts as LiClO4, LiPF6, etc. in an aprotic organic solvent and being interposed between the negative and positive plates to prevent short-circuiting of both plates.
The positive and negative plates are formed in thin sheets or foil shapes, and are piled or wound spirally through a intermediary of the separator to form an electric power generating element. The electric power generating element is housed in either a metallic can made of a stainless steel, a nickel plated iron, or lighter aluminum or a battery container made of laminate film, and subsequently an electrolyte is poured into the battery container, which is sealed for fabricating a battery.
Among a variety of characteristics to be generally demanded according to the use conditions, there are a set of high-temperature standing characteristics, which are particularly important characteristics for such a secondary battery as described above. The high temperature standing characteristics are assessed by measuring the swelling degree and the discharge capacity of the battery after the battery in a charged state has been allowed to stand for a specified duration in an environment where the temperature is 80° C. or above.
There are available many methods for improving the high temperature standing characteristics, among which are, for such a lithium secondary battery as described above, a method in which a solvent having a high boiling point and a low vapor pressure is used, and a method in which the decomposition of the nonaqueous electrolyte on the surfaces of the positive and negative plates is suppressed.
However, as in the former case, when a solvent having a high boiling point and a low vapor pressure is used, there occurs a problem that generally the viscosity of such a solvent is high and the electric conductivity of the nonaqueous electrolyte is lowered, and hence the discharge characteristics of the battery are lowered, etc. Accordingly, desirable is a method in which a small amount of an additive is added to the nonaqueous electrolyte in order not to degrade the electric conductivity of the nonaqueous electrolyte, and a satisfactory coating film is made to be formed on the positive or negative plate in order to kinetically stabilize the nonaqueous electrolyte.
Nowadays, nonaqueous batteries are more frequently adopted for use in a variety of electronic appliances not only in the atmospheric temperature environment but also in a variety of environments of from low to high temperatures. In particular, for example, a cellular telephone left in a sun-heated car makes the nonaqueous electrolyte secondary battery built therein be exposed to a high temperature environment. Thus, the characteristics in the high temperature environments of a nonaqueous electrolyte secondary battery becomes important among the characteristics thereof.
For example, a lithium secondary battery for use in a cellular telephone is required to be small in the swelling degree thereof when it is allowed to stand at 80° C. for a specified duration. However, when a conventional battery described above is left at a high temperature for a long period of time, the battery sometimes gets swollen owing to the gas generated inside the battery. In addition, in the late years, with increasing energy densities of a battery, a battery is demanded to be lighter and thinner, which constitutes a situation in which a battery tends to get more easily swollen.
As a measure to suppress the swelling of a battery when it is allowed to stand at a high temperature, there is a method in which a small amount of an additive is added to the nonaqueous electrolyte for the purpose of suppressing the decomposition of the nonaqueous electrolyte on the plates. For example, as Japanese Patent Laid-Open No. 2002-15768 discloses, there is known a method in which vinylene carbonate is added to the nonaqueous electrolyte of a nonaqueous electrolyte secondary battery. According to this method, it becomes possible to suppress the swelling of the battery when it is allowed to stand at a high temperature, while improving the discharge characteristics. However, even with such a method, the swelling of the battery cannot be suppressed sufficiently, and hence it is desirable to develop an additive having a further efficient suppressing effect.